1. Field of the Art
The present invention relates to a method for bleaching oxygen delignified cellulose-containing pulp such as sulphate, sodium and sulphite pulps with ozone and peroxide.
2. Statement of Prior Art
Usual bleaching processes for producing completely bleached pulps (degree of whiteness=88% MGO) comprises one or two chlorinating stages in which elementary chlorine is used and in most cases one or two additional hypochlorite stages and one or two chlorine dioxide stages. In the chlorinating stage in which generally 4-8% Cl.sub.2 calculated of the quantity of the pulp, is used, large quantities of hydrochloric acid are formed, which entails that the backwater contains large quantities of this acid or sodium chloride if the acid is neutralized. Besides, the backwater will contain organic chlorine compounds. To a lesser degree this is also the case in the hypochlorite bleaching and the chlorine dioxide bleaching.
These organic substances and above all the organic substances which are dissolved during the bleaching of the pulp, mean a heavy charge on the water courses and the lakes into which the backwater is discharged. Besides, the chlorine containing liquids are strongly corrosive and will create large difficulties in a recovery system.
In this connection large efforts have been made to arrive at a bleaching process in which the stage of chlorine bleaching may be replaced by a stage of oxygen bleaching.
However, up till now no method is known in which, through the application of oxygen, the use of chlorine containing bleaching agents (chlorine dioxide) may be avoided for achieving completely bleached qualities.
As examples of conventional bleaching sequences the following stages of treatment may be referred to: C/E/H, C/E/H/E/H, C/E/H/H, C/E/H/D, C/E/D/E/D, C/E/H/D/P, C/E/H/D/E/D, C/E/H/E/D and C/E/H/D/E/H.
Recently one has tried to reduce the quantity of chlorine containing bleaching agent by replacing chlorine (C) with oxygen (O.sub.2), but chlorine dioxide must still be used to achieve completely bleached qualities. Examples of such sequences of treatment are as follows:
A/O.sub.2 /D/E/D, O.sub.2 /D/E/D, O.sub.2 /D/E/D, A/O.sub.2 /D/P/D.
In the above-mentioned sequences the designations mean: C: chlorine, E: alkali, H: hypochlorite, D: chlorine dioxide, P: peroxide, O.sub.2 : oxygen, A: acid wash, Z: ozone.
From Swedish Patent Specification No. 7305671-5 there is known a 4 to 5-stage chlorine-free bleaching process with alternating use of peroxide and peracid. However, such a process is very costly with today's prices of the chemicals included in the process and will hardly find practical application.